Characterization and operation of a multi-channel Condensation Particle Counter (mc-CPC) for aircraft-based measurements

Abstract. Field measurements of aerosol number concentration and aerosol size distribution in the upper troposphere and lowermost stratosphere (UTLS) are crucial for understanding the influence of processes such as new particle formation (NPF) on aerosol budgets, cloud formation and climate. In this study, we present the multi-channel Condensation Particle Counter (mc-CPC) that was designed and constructed for airborne measurements and tested during the TPEx campaign onboard a Learjet in 2024. The instrument uses FC-43 (C₁₂F₂₇N) as the working fluid and consists of three individual CPCs (Grimm SKY-CPC), a pressure regulation system and a common inlet. By varying the temperature difference (ΔT) between each pair of saturator and condenser, the individual cutoff diameters (d₅₀) of each CPC can be adjusted. For the cases presented here, we typically operated two of the CPCs at a ΔT of 36 °C for a direct comparison while the other CPC was set to a ΔT of 15 °C. Two independent calibration setups were used to determine the cutoff and size-dependent counting efficiency of the mc-CPC at various internal and external CPC pressure levels. The experiments in the laboratory showed that the cutoffs of the individual channels were rather independent of the external pressure p_external and only slightly dependent on the internal CPC pressure p_CPC, at least for a p_CPC range between 200–350 hPa. A large fraction of flights during TPEx were conducted at an internal pressure of 250 hPa, and therefore the cutoff determined at 250 hPa was used as a fixed value for all internal pressures. For channel 1 and 2 that were operated at the same ΔT, this gave a d₅₀ of 11.3 (±1.0) nm and 12.3 (±1.1) nm, respectively. Channel 3 was set to ΔT = 15°C and a cutoff diameter of 14.9 (±1.3) nm was determined. In an internal pressure range between 200 hPa and 400 hPa the cutoffs decreased slightly with increasing p_CPC. Furthermore, our measurements also indicate that the cutoffs are not influenced by varying sample flows. The mc-CPC was operated for the first time on an aircraft during the TPEx campaign (TropoPause composition gradients and mixing Experiment) in June 2024. We present the first measurements of one research flight and discuss the uncertainties of the collected aerosol data.